In recent years, an audio system such as a two-way stereo system and a home theater system has become widespread. In addition to two speakers respectively providing right and left outputs of a stereo sound (also hereinafter referred to as right and left speakers), the audio system is equipped with a speaker outputting a sound in which only the low-frequency sound is emphasized, for example, in order to enhance sound effects. Such an audio system requires a plurality of power amplifier circuits for respective speakers in order to output audio signals to a plurality of speakers. A so-called digital power amplifier (Class D amplifier) circuit which performs pulse modulation such as PWM and PDM to output a pulse is widely used as the above-described power amplifier circuit due to its high efficiency (for example, Japanese Patent Laying-Open No. 2002-299968 (Patent Documents 1)).
FIG. 2 is a block diagram showing an example of a conventional audio system using a digital power amplifier circuit. Referring to this figure, the audio system includes audio power amplifier ICs 101, 101a and 101b, and speakers 141, 141a, 141b and 142.
This audio system is referred to as a so-called two-way stereo system. In other words, the audio system outputs a stereo sound through audio power amplifier IC 101 from left and right speakers 141 and 142 based on stereo audio signals Lin and Rin input from outside, and outputs a stereo sound in which only the low-frequency sound is emphasized through audio power amplifier ICs 101a and 101b for low-frequency sounds from left and right speakers 141a and 141b for low-frequency sounds.
Furthermore, low-frequency-sound speakers 141a and 141b have a so-called Balanced Transformerless (BTL) configuration in which two drive inputs are driven in opposite phases to each other in order to increase the output.
Audio power amplifier IC 101, and audio power amplifier ICs 101a and 101b for low-frequency sounds are the same IC. Audio power amplifier ICs 101, 101a and 101b are each provided with digital power amplifier circuits 151 and 152, and a clock generation circuit 110 generating a reference clock BCLK, and the like.
Digital power amplifier circuits 151 and 152 each perform pulse modulation based on reference clock BCLK on each audio signal input thereto, and outputs a resultant pulse synchronized with reference clock BCLK.
Audio power amplifier IC 101 drives stereo speakers 141 and 142. Audio power amplifier ICs 101a and 101b for low-frequency sounds drive speakers 141a and 141b having the BTL configuration, respectively.
Patent Document 1: Japanese Patent Laying-Open No. 2002-299968